frankenword = Frankenstein + word
Like Frankenstein's monster, frankenwords are "unnatural" creations
formed by sewing together bits and pieces of other, separate things --
words in one case, body parts in another.
Frankenword is nearly synonymous with portmanteau. Frankenword, however, goes a little beyond squishing two words together and encompasses the mixing of roots and affixes that aren't necessarily recognizeable as separate words. For example, tacking a Greek suffix (like -ize) onto the end of a Latin root word (like public) to create a word in English (publicize) that doesn't seem to have an etymological home.
But your average portmanteau is also considered a frankenword.
Jan Freeman wrote about frankenwords in more depth (and with more beauty) in the Boston Globe on Sunday, starting with various -athons (e.g., walkathons) and moving on to electrocute, automobile, various -aholics, and others. It's certainly worth a read.